Mr Yanukovych offered opposition chiefs Arseniy Yatsenyuk and Vitali Klitschko the posts of prime minister and deputy prime minister in a new government.

Video:Ukraine Clashes Amid Calls For Calm

But world boxing champion Mr Klitschko said: "Our demand is the holding of presidential elections this year... "

"We will not yield, but talks will continue," he told tens of thousands of protesters in Independence Square in Kiev.

Mr Yatsenyuk, head of the Fatherland party, said the opposition was prepared to take on responsibility, but added: "The people will determine the power in Ukraine. We will not step down from our position."

In recent talks between Mr Yanukovych and the opposition, both sides agreed that the protests and police presence in Kiev would be scaled down, raising hopes of a resolution to the crisis.

Mr Yanukovych also said he was willing to consider changes to the constitution that would reduce the presidency's huge powers.

He also promised to consider changes to anti-protest laws passed by parliament on January 16 which sparked the latest crisis.

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Gallery: Latest Ukraine Protest Pictures

An anti-government protester gets ready to throw a Molotov cocktail at the police lines.

Until the past week, protests had centred mainly in Kiev, with only smaller rallies elsewhere.

But following the latest clashes, a series of government buildings have been seized in the west of the country, where support for Mr Yanukovych is weak.

Protesters clashed with riot police as they forced their way into a regional administration office in the town of Vinnytsia, around 120 miles from Kiev.

And about 100 anti-government activists rushed the country's energy ministry in Kiev, where clashes with police have continued.

Energy Minister Eduard Stavytsky said their actions had threatened the country's entire power supply.

Protesters also continue to occupy Kiev city hall, which they have turned into a makeshift headquarters.

Video:Arnie Wishes Ukraine Protests Luck

The rallies began in November last year after Mr Yanukovych scrapped a treaty with the European Union in favour of a bail-out loan from Russia, following lobbying by Russian President Vladimir Putin.

Swedish foreign minister Carl Bildt suggested Mr Yanukovych was losing control over his country. He posted a map of Ukraine on Twitter showing which regions had been hit by protests.

"If Kiev regime tries a military solution to this situation, it will be very bloody and it will fail," he tweeted.